1. Field of the Invention
The present invention relates generally to gas turbine combustors, and more particularly, to a structure of a burner in a gas turbine combustor constructed to achieve stable combustion of flame-retardant low-Btu gases.
2. Description of the Related Art
In general, fuels of lower heating values burn more slowly, since they are low in flame temperature and hence in burning velocity as well, compared with liquefied natural gas (LNG) which is a principal fuel of gas turbines. Another major feature of these fuels is their low levels of NOx emissions associated with combustion. Typical examples of these low-Btu gases include blast furnace gases. Blast furnace gases are off-gases stemming from blast furnaces in a steel production process, and needs for utilizing these gases as gas turbine fuels, are growing in recent years.
Blast furnace gases are flame-retardant gases that contain carbon monoxide (CO) and hydrogen (H2) as their principal flammable gas, and are heavily laden with nitrogen (N2) and carbon dioxide (CO2) as well. These properties make it difficult to operate a gas turbine in its full load range by means only of a single blast-furnace gas as its fuel, from an ignition phase. To implement stable combustion of the blast-furnace gas in a partial load range of low combustion temperatures from the ignition phase, it is necessary to carburet the gas by mixing a hydrogen-containing coke oven gas or equivalent into the blast furnace gas, or to provide a liquid fuel or any other appropriate start-up fuel separately. In addition, since the stable combustion of flame-retardant gases is required, gas turbine combustors commonly employ a diffuse combustion scheme in which fuel and air are supplied from independent flow channels.
Meanwhile, a structural example of a low-Btu gas-fired burner is disclosed in JP-1993-86902-A. This burner employs a structure with a start-up fuel nozzle provided in a radially central section of the burner, gas injection holes arranged around the fuel nozzle, and gas injection holes and air injection holes further arranged at alternate positions around the former gas injection holes. The burner is targeted for a low-Btu gas heavily laden with N2, such as a coal gasification syngas.
In general, in a burner using a swirling flow to stabilize a flame, a circulating gas region in which combustion gases circulate to impart heat to the fuel and air blasted from the burner needs to be formed in a neighborhood of its radially central section to stabilize the flame. The burner in JP-1993-86902-A actively utilizes a low-Btu gas to form the circulating gas region. This burner, which includes gas injection holes arranged only around an inner swirler, is constructed so that when a large portion of fuel is supplied to the inner swirler, a strong swirling flow is formed by utilizing a momentum of a large amount of low-Btu gas to enhance flame stability.